Liver-X-Receptors (LXRs) are members of the nuclear hormone receptor superfamily. The LXRs are activated by endogenous oxysterols and regulate the transcription of genes controlling multiple metabolic pathways. Two subtypes, LXRalpha and LXRbeta, have been described (Willy et al., Genes Dev. 1995, 9:1033–45; Song et al., Proc Natl Acad Sci USA. 1994, 91:10809–13). LXRbeta is ubiquitously expressed, while LXRalpha is predominantly expressed in cholesterol metabolizing tissues such as the liver, adipose, intestine and macrophage. The LXRs modulate a variety of physiological responses including regulation of cholesterol absorption, cholesterol elimination (bile acid synthesis), and transport of cholesterol from peripheral tissues via plasma lipoproteins to the liver. The LXRs are also involved in glucose metabolism, cholesterol metabolism in the brain, cell differentiation, and inflammation.
At present, approximately half of all patients with coronary artery disease have low concentrations of plasma high-density lipoprotein cholesterol (HDL-C). The atheroprotective function of HDL was first highlighted almost 25 years ago and stimulated exploration of the genetic and environmental factors that influence HDL-C levels (Miller N E., Lipids 1978,13:914–9). The protective function of HDL derives from its role in a process termed reverse cholesterol transport. HDL mediates the removal of cholesterol from cells in peripheral tissues, including macrophage foam cells in the atherosclerotic lesions of the arterial wall. HDL delivers its cholesterol to the liver and sterol-metabolizing organs for conversion to bile and elimination in feces. Studies have shown that HDL-C levels are predictive of coronary artery disease risk independently of low-density lipoprotein cholesterol (LDL-C) levels (Gordon et al., Am J Med. 1977, 62:707–14).
At present, the estimated age-adjusted prevalence among Americans age 20 and older who have HDL-C of less than 35 mg/dl is 16% (males) and 5.7% (females). A substantial increase of HDL-C is currently achieved by treatment with niacin in various formulations. However, the substantial unfavorable side-effects limit the therapeutic potential of this approach.
It has been observed that as many as 90% of the 14 million diagnosed type 2 diabetic patients in the United States are overweight or obese, and a high proportion of type 2 diabetic patients have abnormal concentrations of lipoproteins. Studies have shown that the prevalence of total cholesterol>240 mg/dl is 37% in diabetic men and 44% in women. The rates for LDL-C>160 mg/dl are 31% and 44%, and for HDL-C<35 mg/dl are 28% and 11%, in diabetic men and women respectively. Diabetes is a disease in which a patient's ability to control glucose levels in blood is decreased because of partial impairment in response to the action of insulin. Type II diabetes (T2D) is also called non-insulin dependent diabetes mellitus (NIDDM) and has been shown to afflict 80–90% of all diabetic patients in developed countries. In T2D, the pancreatic Islets of Langerhans continue to produce insulin. However, the target organs for insulin action, mainly muscle, liver and adipose tissue, exhibit a profound resistance to insulin stimulation. The body continues to compensate by producing unphysiologically high levels of insulin, which ultimately decreases in the later stages of the disease, due to exhaustion and failure of pancreatic insulin-producing capacity. Thus, T2D is a cardiovascular-metabolic syndrome associated with multiple co-morbidities, including insulin resistance, dyslipidemia, hypertension, endothelial dysfunction and inflammatory atherosclerosis.
The first line of treatment for dyslipidemia and diabetes at present generally involves a low-fat and low-glucose diet, exercise and weight loss. However, compliance can be moderate, and as the disease progresses, treatment of the various metabolic deficiencies becomes necessary with lipid-modulating agents such as statins and fibrates for dyslipidemia, and hypoglycemic drugs, e.g. sulfonylureas, metformin, or insulin sensitizers of the thiazolidinedione (TZD) class of PPARγ-agonists, for insulin resistance. Recent studies provide evidence that modulators of LXRs would result in compounds with enhanced therapeutic potential, and as such, modulators of LXRs should improve the plasma lipid profile, and raise HDL-C levels (Lund et al., Arterioscler. Thromb. Vasc. Biol. 2003, 23:1169–77). LXRs are also known to control the efflux of cholesterol from the macrophage foam cell of the atherosclerotic lesion, and agonists of LXRs have been shown to be atheroprotective (Joseph and Tontonoz, Curr. Opin. Pharmacol. 2003, 3:192–7). Thus, modulators of LXRs would be effective treatments for the atherosclerotic disease which underlies the cardiovascular morbidity and mortality of stroke and heart disease. Recent observations also suggest that there is an independent LXR mediated effect on insulin-sensitization in addition to its role in atheroprotection (Cao et al., J Biol Chem. 2003, 278:1131–6). Thus LXR modulators can also show superior therapeutic efficacy on HDL-raising and atheroprotection, with additional effects on diabetes, compared to current therapies.